U.S. patent application number 14/235927 was filed with the patent office on 2014-07-10 for stainless steel for fuel cell separator.
This patent application is currently assigned to JFE STEEL CORPORATION. The applicant listed for this patent is Shinsuke Ide, Tomohiro Ishii, Shin Ishikawa, Noriko Makiishi, Masayasu Nagoshi, Hisato Noro. Invention is credited to Shinsuke Ide, Tomohiro Ishii, Shin Ishikawa, Noriko Makiishi, Masayasu Nagoshi, Hisato Noro.
Application Number | 20140193668 14/235927 |
Document ID | / |
Family ID | 47628873 |
Filed Date | 2014-07-10 |
United States Patent
Application |
20140193668 |
Kind Code |
A1 |
Makiishi; Noriko ; et
al. |
July 10, 2014 |
STAINLESS STEEL FOR FUEL CELL SEPARATOR
Abstract
Stainless steel for fuel cell separators are provided which
exhibit stable contact resistance characteristics and excellent
practical utility. The stainless steel contains not less than 16
mass % of Cr. When the surface configuration of the stainless steel
is analyzed with a scanning electron microscope at a spatial
resolution of not more than 0.1 .mu.m, the surface modulus is not
less than 1.02. Preferably, the chemical composition further
includes C: not more than 0.03%, Si: not more than 1.0%, Mn: not
more than 1.0%, S: not more than 0.01%, P: not more than 0.05%, Al:
not more than 0.20%, N: not more than 0.03%, Cr: 16 to 40%, and one
or more of Ni: not more than 20%, Cu: not more than 0.6% and Mo:
not more than 2.5%, the balance being Fe and inevitable
impurities.
Inventors: |
Makiishi; Noriko; (Chiba,
JP) ; Noro; Hisato; (Kawasaki, JP) ; Ishikawa;
Shin; (Chiba, JP) ; Ide; Shinsuke; (Chiba,
JP) ; Ishii; Tomohiro; (Chiba, JP) ; Nagoshi;
Masayasu; (Chiba, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Makiishi; Noriko
Noro; Hisato
Ishikawa; Shin
Ide; Shinsuke
Ishii; Tomohiro
Nagoshi; Masayasu |
Chiba
Kawasaki
Chiba
Chiba
Chiba
Chiba |
|
JP
JP
JP
JP
JP
JP |
|
|
Assignee: |
JFE STEEL CORPORATION
Chiyoda-ku ,Tokyo
JP
|
Family ID: |
47628873 |
Appl. No.: |
14/235927 |
Filed: |
July 25, 2012 |
PCT Filed: |
July 25, 2012 |
PCT NO: |
PCT/JP2012/004740 |
371 Date: |
January 29, 2014 |
Current U.S.
Class: |
428/687 |
Current CPC
Class: |
C22C 38/40 20130101;
H01M 8/021 20130101; C22C 38/04 20130101; Y02E 60/50 20130101; C21D
6/004 20130101; C22C 38/28 20130101; C22C 38/004 20130101; C22C
38/06 20130101; C22C 38/22 20130101; C22C 38/001 20130101; C22C
38/26 20130101; C21D 9/46 20130101; C23G 1/086 20130101; C25F 1/00
20130101; Y10T 428/12993 20150115; C22C 38/02 20130101; Y02P 70/50
20151101 |
Class at
Publication: |
428/687 |
International
Class: |
H01M 8/02 20060101
H01M008/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 29, 2011 |
JP |
2011-166579 |
Claims
1. A stainless steel for fuel cell separators, wherein the
stainless steel contains not less than 16 mass % of Cr and has a
surface modulus of not less than 1.02, the surface modulus being
obtained by a measurement of surface configuration with respect to
the surface of the stainless steel with a scanning electron
microscope at a spatial resolution of not more than 0.1 .mu.m.
2. The stainless steel for fuel cell separators according to claim
1, wherein the surface modulus is not less than 1.04.
3. The stainless steel for fuel cell separators according to claim
1, wherein the stainless steel further contains, by mass, C: not
more than 0.03%, Si: not more than 1.0%, Mn: not more than 1.0%, S:
not more than 0.01%, P: not more than 0.05%, Al: not more than
0.20%, N: not more than 0.03%, Cr: 16 to 40%, and one or more of
Ni: not more than 20%, Cu: not more than 0.6% and Mo: not more than
2.5%, the balance being Fe and inevitable impurities.
4. The stainless steel for fuel cell separators according to claim
1, wherein the stainless steel further contains not more than 1.0%
by mass of a total of one or more of Nb, Ti and Zr.
5. The stainless steel for fuel cell separators according to claim
2, wherein the stainless steel further contains not more than 1.0%
by mass of a total of one or more of Nb, Ti and Zr.
6. The stainless steel for fuel cell separators according to claim
3, wherein the stainless steel further contains not more than 1.0%
by mass of a total of one or more of Nb, Ti and Zr.
7. The stainless steel for fuel cell separators according to claim
2, wherein the stainless steel further contains, by mass, C: not
more than 0.03%, Si: not more than 1.0%, Mn: not more than 1.0%, S:
not more than 0.01%, P: not more than 0.05%, Al: not more than
0.20%, N: not more than 0.03%, Cr: 16 to 40%, and one or more of
Ni: not more than 20%, Cu: not more than 0.6% and Mo: not more than
2.5%, the balance being Fe and inevitable impurities.
Description
TECHNICAL FIELD
[0001] The present invention relates to stainless steel for fuel
cell separators which is excellent in corrosion resistance and in
contact resistance characteristics.
BACKGROUND ART
[0002] From the viewpoint of environmental conservation, there has
recently been ongoing development of fuel cells that are excellent
in electric power generation efficiency and do not emit carbon
dioxide. A fuel cell produces electricity by the reaction of
hydrogen with oxygen. The basic structure thereof is a sandwich
structure that is composed of an electrolyte membrane, namely, an
ion-exchange membrane, two electrodes (a fuel electrode and an air
electrode), hydrogen and oxygen (air) diffusion layers, and two
separators. Fuel cells developed so far have some types in
accordance with the electrolytes used such as phosphoric acid fuel
cells, molten sodium carbonate fuel cells, solid oxide fuel cells,
alkaline fuel cells and solid polymer fuel cells.
[0003] Of the above fuel cells, solid polymer fuel cells outperform
other types of fuel cells such as molten sodium carbonate fuel
cells and phosphoric acid fuel cells in terms of such
characteristics as (1) a markedly low operating temperature of
about 80.degree. C., (2) reduced weight and size of cell bodies,
and (3) a short transient time, high fuel efficiency and high
output density. Thus, solid polymer fuel cells are one of the most
attractive fuel cells today for use as power sources aboard
electric vehicles as well as compact distributed power sources for
home use and mobile use.
[0004] Solid polymer fuel cells are based on the principle of
obtaining electricity from hydrogen and oxygen via polymer
membranes. A structure thereof is illustrated in FIG. 1. A
membrane-electrode assembly (MEA, having a thickness of several
tens to several hundreds of .mu.m) 1 is a combination of a polymer
membrane and an electrode material such as carbon black carrying a
platinum catalyst and disposed on the front and back sides of the
membrane. This MEA is sandwiched between gas diffusion layers 2 and
3 such as carbon cloth sheets and separators 4 and 5, thereby
forming a unit cell capable of generating an electromotive force
between the separators 4 and 5. Here, the gas diffusion layers are
frequently integrated with the MEA. When used, several tens to
several hundreds of these unit cells are connected in series to
form a fuel cell stack.
[0005] Separators are required to function as partitions separating
unit cells and also as (1) conductors carrying electrons generated,
(2) channels for oxygen (air) and hydrogen (air channels 6 and
hydrogen channels 7 in FIG. 1), and (3) channels for water and
exhaust gas (air channels 6 and hydrogen channels 7 in FIG. 1).
[0006] As described above, the practical use of solid polymer fuel
cells requires separators which exhibit excellent durability and
conductivity. Solid polymer fuel cells that are in practical use at
present utilize separators made of carbonaceous materials such as
graphite. Various other materials such as titanium alloys are under
consideration. However, the carbon separators have drawbacks in
that the separators are easily broken by impact, miniaturization is
difficult, and the formation of channels incurs high processing
costs. In particular, the cost problems are the greatest obstacle
to the wide spreading of fuel cells. Thus, attempts have been made
to replace carbonaceous materials by metal materials, in particular
stainless steel.
[0007] Patent Literature 1 discloses a technique in which a metal
that is easily passivated to form a passivation film is used as a
separator. However, the formation of a passivation film results in
an increase in contact resistance and leads to a decrease in
electric power generation efficiency. Thus, problems with these
metal materials have been indicated such as higher contact
resistance and inferior corrosion resistance as compared to
carbonaceous materials.
[0008] In order to solve these problems, Patent Literature 2
discloses a technique in which the surface of a metallic separator
such as SUS304 is plated with gold to reduce contact resistance and
ensure high output. However, thin gold plating is accompanied by a
difficulty of preventing the occurrence of pinholes. On the other
hand, thick gold plating adds costs.
[0009] Patent Literature 3 discloses a remedy in which separators
with improved conductivity are obtained by dispersing carbon
powders on ferritic stainless steel substrates. However, the use of
carbon powders is a reasonably costly surface treatment for
separators. Further, a problem has been pointed out in which such
surface-treated separators markedly decrease corrosion resistance
in the case where defects such as scratches are caused during
assembling.
[0010] Under the circumstances described above, the present
inventors have filed Patent Literature 4 drawn to a technique in
which a stainless steel material is used as such and the surface
configuration is controlled so as to satisfy both contact
resistance and corrosion resistance. Patent Literature 4 is
directed to a stainless steel sheet characterized in, that the
average spacing between local peak tops in a surface roughness
curve is not more than 0.3 .mu.m, this configuration achieving a
contact resistance of not more than 20 m.OMEGA.cm.sup.2. Although
this technique has made it possible to provide stainless steel
materials as fuel cell separators materials, further improvements
in contact resistance characteristics are desirable from the
viewpoint of fuel cell design and a stable contact resistance of
not more than 10 m.OMEGA.cm.sup.2 is demanded.
CITATION LIST
Patent Literature
[0011] PTL 1: Japanese Unexamined Patent Application Publication
No. 8-180883
[0012] PTL 2: Japanese Unexamined Patent Application Publication
No. 10-228914
[0013] PTL 3: Japanese Unexamined Patent Application Publication
No. 2000-277133
[0014] PTL 4: Japanese Unexamined Patent Application Publication
No. 2005-302713
SUMMARY OF INVENTION
Technical Problem
[0015] The present invention has been made in view of the
circumstances described above. It is therefore an object of the
invention to provide stainless steel for fuel cell separators
excellent in contact resistance characteristics and practical
utility.
Solution to Problem
[0016] The present inventors carried out extensive studies directed
to improving the contact resistance characteristics of stainless
steel for fuel cell separators. As a result, the present inventors
have found that the contact resistance characteristics are improved
by controlling the microscopic surface configuration. The present
inventors have further found that controlling of the chemical
composition is preferable from the viewpoints of ensuring practical
corrosion resistance and mechanical characteristics of stainless
steel as well as costs.
[0017] The present invention is based on the above findings.
Features of the invention are as follows.
[0018] [1] A stainless steel for fuel cell separators wherein the
stainless steel contains not less than 16 mass % of Cr and has a
surface modulus of not less than 1.02, the surface modulus being
obtained by a measurement of surface configuration with respect to
the surface of the stainless steel with a scanning electron
microscope at a spatial resolution of not more than 0.1 .mu.m.
Here, the surface modulus refers to a ratio of the area of
irregular surface to the area of perfectly flat surface.
[0019] [2] The stainless steel for fuel cell separators described
in [1], wherein the surface modulus is not less than 1.04.
[0020] [3] The stainless steel for fuel cell separators described
in [1] or [2], wherein the stainless steel further contains, by
mass, C: not more than 0.03%, Si: not more than 1.0%, Mn: not more
than 1.0%, S: not more than 0.01%, P: not more than 0.05%, Al: not
more than 0.20%, N: not more than 0.03%, Cr: 16 to 40%, and one or
more of Ni: not more than 20%, Cu: not more than 0.6% and Mo: not
more than 2.5%, the balance being Fe and inevitable impurities.
[0021] [4] The stainless steel for fuel cell separators described
in any of [1] to [3], wherein the stainless steel further contains
not more than 1.0% by mass of a total of one or more of Nb, Ti and
Zr.
Advantageous Effects of Invention
[0022] The stainless steel for fuel cell separators according to
the present invention have stable contact resistance
characteristics and excellent practical utility.
[0023] The inventive stainless steel as separators replace
conventional expensive carbon and gold plating to enable the
provision of inexpensive fuel cells and to promote the spread of
fuel cells.
BRIEF DESCRIPTION OF DRAWINGS
[0024] FIG. 1 is a schematic view illustrating a basic structure of
a fuel cell.
[0025] FIG. 2 is a set of SEM (scanning electron microscope) images
illustrating microscopic irregularities on the surface of samples
according to the invention.
[0026] FIG. 3 illustrates a relationship between surface modulus
measured by 3D (three-dimensional)-SEM and contact resistance of
the surface of samples according to the invention.
[0027] FIG. 4 is a set of SEM images illustrating surface
configurations of carbonaceous materials.
DESCRIPTION OF EMBODIMENTS
[0028] Hereinbelow, the present invention will be described in
detail.
[0029] First, stainless steel of interest in the invention will be
described.
[0030] The stainless steel used as the material in the invention is
not particularly limited, and any types of steels may be used as
long as the stainless steel has corrosion resistance required in an
operating environment for fuel cells. However, the stainless steel
needs to contain not less than 16 mass % Cr in order to ensure
basic corrosion resistance. If the Cr content is less than 16 mass
%, the stainless steel cannot endure prolonged use as separators.
The Cr content is preferably not less than 18 mass %. If the Cr
content exceeds 40 mass %, toughness may be lowered at times due to
the precipitation of a phase. Thus, the Cr content is preferably
not more than 40 mass %.
[0031] A particularly preferred chemical composition is described
below. The unit "%" used for the contents of components indicates
mass % unless otherwise mentioned.
[0032] C: Not More Than 0.03%
[0033] The reaction of carbon with chromium in stainless steel
precipitates chromium carbide in the grain boundary, resulting in a
decrease in corrosion resistance. Thus, a lower C content is more
preferable. A marked decrease in corrosion resistance is avoided
when the C content is not more than 0.03%. The C content is more
preferably not more than 0.015%.
[0034] Si: Not More than 1.0%
[0035] Silicon is an effective element for deoxidation and is added
at a stage of smelting of stainless steel. However, excessive
addition causes hardening of the stainless steel sheet and
decreases ductility. Thus, silicon, when added, is preferably
present in a content of not more than 1.0%, and more preferably not
less than 0.01% and not more than 0.6%.
[0036] Mn: Not More Than 1.0%
[0037] Manganese combines to sulfur that has been inevitably mixed
in the stainless steel and thereby effectively decreases the amount
of sulfur dissolved in the stainless steel. Thus, this element is
effective for suppressing the segregation of sulfur at the grain
boundary and for preventing cracking of the steel sheet during hot
rolling. However, adding manganese in excess of 1.0% does not
substantially provide a corresponding increase in the effects. On
the contrary, such excessive addition increases costs. Thus, the
content of manganese, when present, is preferably not more than
1.0%.
[0038] S: Not More Than 0.01%
[0039] Sulfur combines to manganese to form MnS and thereby lowers
corrosion resistance. Thus, the content of this element is
preferably small. A marked decrease in corrosion resistance is
avoided when the S content is not more than 0.01%. Thus, the
content of sulfur, when present, is preferably not more than
0.01%.
[0040] P: Not More Than 0.05%
[0041] Phosphorus causes a decrease in ductility and therefore the
content thereof is desirably small. A marked decrease in ductility
is avoided when the P content is not more than 0.05%. Thus, the
content of phosphorus, when present, is preferably not more than
0.05%.
[0042] Al: Not More Than 0.20%
[0043] Aluminum is used as a deoxidizing element. Excessive
addition of this element causes a decrease in ductility. Thus, the
content of aluminum, when present, is preferably not more than
0.20%.
[0044] N: Not More Than 0.03%
[0045] Nitrogen is an effective element for suppressing the
localized corrosion such as crevice corrosion of stainless steel.
However, the addition of nitrogen in excess of 0.03% requires long
time at a stage of smelting of stainless steel, resulting in a
decrease in productivity and a decrease in the formability of
steel. Thus, nitrogen, when added, is preferably present in a
content of not more than 0.03%.
[0046] One or More of Ni: Not More Than 20%, Cu: Not More Than 0.6%
and Mo: Not More Than 2.5%
[0047] Ni: Not More Than 20%
[0048] Nickel is an element that stabilizes the austenite phase and
is added when austenitic stainless steel is to be produced. If the
Ni content exceeds 20%, such excessive consumption of nickel
increases costs. Thus, the Ni content is preferably not more than
20%.
[0049] Cu: Not More Than 0.6%
[0050] Copper is an effective element for improving the corrosion
resistance of stainless steel. However, the addition in excess of
0.6% not only results in a decrease in hot workability and a
decrease in productivity but also increases costs due to excessive
addition of copper. Thus, the content of copper, when added, is
preferably not more than 0.6%.
[0051] Mo: Not More Than 2.5%
[0052] Molybdenum is an effective element for suppressing the
crevice corrosion of stainless steel. However, the addition in
excess of 2.5% not only results in marked embrittlement of
stainless steel and a decrease in productivity but also increases
costs due to excessive consumption of molybdenum. Thus, the content
of molybdenum, when added, is preferably not more than 2.5%.
[0053] One or More of Nb, Ti and Zr in Total of Not More Than
1.0%
[0054] In the invention, one or more of niobium, titanium and
zirconium may be added in addition to the aforementioned elements
in order to improve grain boundary corrosion resistance. However,
ductility is lowered if the total thereof exceeds 1.0%. Because of
this as well as in order to avoid an increase in costs due to the
addition of these elements, titanium, niobium and zirconium, when
added, are preferably present in a total content of not more than
1.0%.
[0055] The balance is iron and inevitable impurities.
[0056] Next, there will be described characteristics to be met by
the inventive stainless steel for separators.
[0057] The stainless steel of the invention has a surface modulus
of not less than 1.02, and preferably not less than 1.04. The
surface modulus is obtained by a measurement of surface
configuration with respect to the surface with a scanning electron
microscope at a spatial resolution of not more than 0.1 .mu.m.
[0058] FIG. 2 is a set of SEM images illustrating the results of
SEM observation with respect to 1 .mu.m or finer microscopic
surface configurations of a steel sheet A and a steel sheet B with
at least .times.10000 magnification. The steel A represents an
example of ferritic material, and the steel B represents an example
of austenitic material. The steels A and B are both stainless steel
obtained by hot rolling, subsequent annealing at temperatures of
800 to 1100.degree. C., and repeated cold rolling and annealing. In
FIG. 2, the images before treatments indicate the steels as
annealed, and the images after treatments show the steels that have
been electrolytically treated in a 3% sulfuric acid solution at
55.degree. C. and thereafter immersed in a hydrofluoric acid
solution after being annealed.
[0059] From FIG. 2, it is apparent that the microscopic
irregularities on the surface of the steels A and B became
significantly finer and numerous after the treatments compared to
before the treatments.
[0060] Separately, the contact resistances of the surface of the
steels A and B were measured before and after the treatments. The
contact resistances of the steels A and B were both not less than
100 m.OMEGA.cm.sup.2 before the treatments, but were both reduced
to not more than 10 m.OMEGA.cm.sup.2 after the treatments. In the
measurement of contact resistance, carbon paper CP120 manufactured
by TORAY INDUSTRIES, INC. was used, and the resistance at the
interface of the carbon paper CP120 and the steel A or B in contact
with each other under a load of 20 kGf/cm.sup.2 was measured.
[0061] Next, in order to quantify the microscopic irregularities
shown in FIG. 2, the surface moduli were obtained using a 3D-SEM
fitted with a field-emission electron gun. The surface modulus is a
ratio of the area of irregular surface to the area of perfectly
flat surface. The surface moduli were determined in the following
manner. SEM data was obtained by analyzing the thinly Au-deposited
surface at an accelerating voltage of 5 kV. From the obtained
3D-SEM images, the surface modulus was calculated according to the
equation: surface modulus=(1+Sdr/100) wherein Sdr was the developed
interfacial area ratio which was one of the 3D parameters. To
eliminate influences of grain boundaries, at least three fields of
view of regions free of crystal grain boundaries were analyzed with
.times.20000 magnification, the results being averaged to give the
surface modulus.
[0062] The measurement resulted in surface moduli of the steels A
and B before and after the treatments ranging from 1.01 to 1.05. As
illustrated in FIG. 3, it has been found that the contact
resistance tends to be lower with increasing surface modulus. The
reason for this tendency is considered to be as follows. FIG. 4
illustrates surface SEM images of carbonaceous materials, namely,
carbon paper and carbon cloth. These carbonaceous materials contact
stainless steel sheets via dispersed carbon fibers having a
diameter of about 10 .mu.m. In order to ensure conductivity at
contacts, a pressure needs to be applied to the contact points. The
present inventors assume that a larger number of microscopic
irregularities, that is, a higher surface modulus provides a larger
number of contact points at which a high pressure for establishing
contacts can be easily obtained, thus achieving a further decrease
in contact resistance. Thus, the reason for the above tendency is
probably because the inventive configuration produces an effect of
increasing the contact area. From FIG. 3, a contact resistance of
not more than 10 m.OMEGA.cm.sup.2 is achieved by controlling the
surface modulus to be not less than 1.02. A surface modulus of not
less than 1.04 is shown to provide a still lower contact
resistance.
[0063] Based on the above results, the present invention provides
that when the surface configuration of the stainless steel is
analyzed with a scanning electron microscope at a spatial
resolution of not more than 0.1 .mu.m, the surface modulus is not
less than 1.02, and preferably not less than 1.04. The surface
modulus may be regulated to fall in the inventive range by
controlling treatment conditions such as by changing the treatment
time and the temperature of the treatment liquid when the annealed
steel is electrolytically treated in an acidic solution and is
thereafter immersed in an acidic solution. To obtain a higher
surface modulus, a higher temperature and a longer time are
advantageous.
[0064] The measurement of surface modulus is not particularly
limited. For example, the surface modulus may be obtained using a
3D-SEM fitted with a field-emission electron gun. For example,
other devices such as an atomic force microscope may be utilized.
Contact roughness meters are incapable of this irregularity
evaluation because the diameter of the probe tip is on the order of
micrometers. It is also possible to evaluate with a cross-sectional
TEM (transmission electron microscope). However, a 3D-SEM is
considered to be most suited to obtain representativeness from the
evaluation of many fields of view.
[0065] The stainless steel for fuel cell separators having stable
contact resistance characteristics and excellent practical utility
may be produced by any conventional methods without limitation.
Preferred production conditions are described below.
[0066] A slab conditioned to have a preferred chemical composition
is heated to a temperature of not less than 1100.degree. C.,
thereafter hot rolled, annealed at temperatures of 800 to
1100.degree. C., and subsequently subjected to cycles of cold
rolling and annealing to give a stainless steel. The sheet
thickness of the obtained stainless steel sheet is suitably about
0.02 to 0.8 mm. After finish annealing, the steel sheet is
preferably subjected to electrolytic treatment and acidizing. As an
example of electrolytic conditions, the electrolytic treatment may
be performed in a 3% sulfuric acid (H.sub.2SO.sub.4) bath at 2
A/dm.sup.2 and 55.degree. C. for 30 seconds. As an example of the
acidizing, the steel sheet may be immersed in a HCl:H.sub.2O=1:3
(by volume) liquid mixture at 50.degree. C. for 30 seconds.
EXAMPLE 1
[0067] Steels having a chemical composition described in Table 1
were smelted in a vacuum melting furnace. The obtained steel ingots
were heated to 1200.degree. C. and were then hot rolled to give hot
rolled sheets with a sheet thickness of 5 mm. The hot rolled sheets
were annealed at 900.degree. C., descaled by pickling, and
subjected to cycles of cold rolling, annealing and pickling. Thus,
stainless steel sheets with a sheet thickness of 0.2 mm were
produced.
[0068] Subsequently, the steel sheets were annealed, pretreated (by
electrolytic treatment or pickling) under conditions described in
Table 2, and acidized by being immersed in a pickling solution. The
electrolytic treatment was carried out in a bath described in Table
2 at a solution temperature of 55.degree. C. and a current density
of 2 A/dm.sup.2 for a treatment time of 30 seconds. The pickling
was performed with a solution described in Table 2 at a solution
temperature of 55.degree. C. for a treatment time of 30 seconds.
The acidizing was carried out in a solution at a bath temperature
described in Table 2 for 120 seconds.
[0069] The stainless steels obtained above were tested to measure
the contact resistance and were analyzed by 3D-SEM to determine the
configuration and to evaluate the surface modulus.
[0070] In the measurement of contact resistance, carbon paper CP120
manufactured by TORAY INDUSTRIES, INC. was used, and the resistance
at the interface of the carbon paper CP120 and the steel in contact
with each other under a load of 20 kGf/cm.sup.2 was measured.
[0071] The 3D-SEM measurement was performed with ERA-8800FE
manufactured by Elionix at an accelerating voltage of 5 kV. The
Elionix's ERA-8800FE was equipped with four secondary electron
detectors directed to the sample direction and was capable of
displaying images emphasizing differences in chemical compositions
as well as images reflecting irregularities in a specific direction
based on additive signals and subtractive signals of secondary
electrons.
[0072] The obtained SEM images were processed with an attached
image processing software (three-dimensional surface configuration
analysis software "SUMMIT"), and the surface modulus was calculated
according to the equation: surface modulus=(1+Sdr/100). In the
measurement, a 6 .mu.m.times.4.5 .mu.m region was divided into
pixels with 0.01 .mu.m intervals. In the image processing,
calculations were made while performing 1/2.lamda. second-order
Gaussian highpass filtering. At least five fields of view of each
sample were observed, the results being averaged. Images including
grain boundaries were excluded because the unevenness at the grain
boundaries would greatly affect the results. That is, the
calculations involved data obtained within grains.
[0073] The results are described in Table 3.
TABLE-US-00001 TABLE 1 Components (mass %) Steel No. C N Si Mn P S
Cr Ni Mo Cu Nb Al Ti Remarks 1 0.0083 0.0051 0.18 0.20 0.026 0.002
29.5 -- 1.96 -- 0.11 0.01 0.12 Inv. Ex. 2 0.0079 0.0049 0.17 0.25
0.015 0.001 21.1 -- -- 0.49 -- 0.03 0.32 Inv. Ex. 3 0.0081 0.0043
0.22 0.45 0.025 0.003 18.1 8.2 -- -- -- 0.05 Inv. Ex.
TABLE-US-00002 TABLE 2 Pretreatment Acidizing Electrolytic Temper-
Conditions treatment Pickling Bath ature A Conditions X1 --
Conditions Z1 55.degree. C. B Conditions X2 -- Conditions Z1
55.degree. C. C Conditions X2 -- Conditions Z2 55.degree. C. D
Conditions X2 -- Conditions Z2 45.degree. C. E Conditions X2 --
Conditions Z3 55.degree. C. F -- Conditions Y1 Conditions Z2
55.degree. C. G -- -- Conditions Z2 55.degree. C. H Conditions X1
-- -- -- Conditions X1: electrolytic treatment in 3%
H.sub.2SO.sub.4 bath Conditions X2: electrolytic treatment in 15%
Na.sub.2SO.sub.4 bath Conditions Y1: pickling with HCl(1 + 3)
Conditions Z1: HNO3 10% + HF3% Conditions Z2: HF10% Conditions Z3:
HF11% + HNO3 10%
TABLE-US-00003 TABLE 3 Surface modulus Not less Contact Test Steel
than Not less resistance No. No. Treatment 1.02 than 1.04 (m.OMEGA.
cm.sup.2) Remarks 1 1 A .largecircle. .largecircle. 5.9 Inv. Ex. 2
1 B X X 11.3 Comp. Ex. 3 1 C .largecircle. X 9.9 Inv. Ex. 4 1 D X X
37.6 Comp. Ex. 5 1 E X X 23.4 Comp. Ex. 6 1 G X X 15.3 Comp. Ex. 7
1 -- X X 136.5 Comp. Ex. 8 2 A .largecircle. X 9.5 Inv. Ex. 9 2 B
.largecircle. .largecircle. 4.5 Inv. Ex. 10 2 C .largecircle.
.largecircle. 5.6 Inv. Ex. 11 2 D .largecircle. X 8.9 Inv. Ex. 12 2
E X X 11.3 Comp. Ex. 13 2 F .largecircle. X 9.7 Inv. Ex. 14 2 G X X
12.1 Comp. Ex. 15 2 H X X 156.0 Comp. Ex. 16 2 -- X X 206.0 Comp.
Ex. 17 3 B .largecircle. X 9.3 Inv. Ex. 18 3 C .largecircle.
.largecircle. 5.7 Inv. Ex. 19 3 D .largecircle. X 8.8 Inv. Ex. 20 3
E X X 34.0 Comp. Ex. 21 3 F .largecircle. X 8.9 Inv. Ex. 22 3 G
.largecircle. X 9.6 Inv. Ex. 23 3 H X X 154.7 Comp. Ex. 24 3 -- X X
306.7 Comp. Ex.
[0074] From Table 3, it has been demonstrated that Inventive
Examples in which the surface modulus was not less than 1.02
achieved a contact resistance of not more than 10 m.OMEGA.cm.sup.2.
Further, many of the steel sheets with a surface modulus of not
less than 1.04 exhibited a contact resistance of not more than 8
m.OMEGA.cm.sup.2 and were shown to be further enhanced in terms of
contact resistance characteristics.
EXAMPLE 2
[0075] Of the 0.2 mm thick stainless steel sheets used in EXAMPLE
1, the sheets of the steels Nos. 2 and 3 described in Table 1 were
utilized. As a pretreatment, the steel sheets were electrolytically
treated in a 3% sulfuric acid solution. The solution temperature
was 55.degree. C., the current density was 2 A/dm.sup.2, and the
treatment time was 30 seconds. The steel sheets were then acidized
by being immersed in a solution mixture of 5% hydrofluoric acid and
1% nitric acid for the steel No. 2 and in a 5% hydrofluoric acid
solution for the steel No. 3. The temperature of both the acid
solutions was 55.degree. C., and the immersion time was 40 seconds
to 120 seconds. For comparison, samples were prepared without acid
immersion. The surface of the obtained samples was tested to
measure the contact resistance and was analyzed by 3D-SEM to
determine the configuration and to evaluate the surface modulus.
These measurements and data analysis were carried out by the same
methods as in EXAMPLE 1.
[0076] The results are described in Table 4.
TABLE-US-00004 TABLE 4 Electro- Immer- Contact Test Steel lytic
sion Surface resistance No. No. treatment time modulus (m.OMEGA.
cm.sup.2) Remarks 25 2 Performed -- 1.012 15.0 Comp. Ex. 26 2
Performed 40 1.014 11.4 Comp. Ex. 27 2 Performed 60 1.056 4.7 Inv.
Ex. 28 2 Performed 120 1.049 5.2 Inv. Ex. 29 3 Performed -- 1.010
44.0 Comp. Ex. 30 3 Performed 40 1.014 13.0 Comp. Ex. 31 3
Performed 60 1.023 8.2 Inv. Ex. 32 3 Performed 120 1.045 4.1 Inv.
Ex.
[0077] Under the conditions adopted in this EXAMPLE, a surface
modulus of not less than 1.02 was obtained when the acid immersion
time was 60 seconds or more, resulting in a contact resistance of
not more than 10 m.OMEGA.cm.sup.2. The steel No. 2 treated under
these immersion conditions attained a surface modulus of not less
than 1.04 and a contact resistance of not more than 8
m.OMEGA.cm.sup.2. On the other hand, the steel No. 3 achieved a
surface modulus of not less than 1.04 and a contact resistance of
not more than 8 m.OMEGA.cm.sup.2 when immersed in the acid for 120
seconds.
[0078] As demonstrated above, stainless steel for fuel cell
separators exhibiting excellent contact resistance characteristics
are realized by configuring the stainless steel such that when the
surface configuration is analyzed with a scanning electron
microscope at a spatial resolution of not more than 0.1 .mu.m, the
surface modulus is not less than 1.02, and preferably not less than
1.04.
REFERENCE SIGNS LIST
[0079] 1 MEMBRANE-ELECTRODE ASSEMBLY
[0080] 2 GAS DIFFUSION LAYER
[0081] 3 GAS DIFFUSION LAYER
[0082] 4 SEPARATOR
[0083] 5 SEPARATOR
[0084] 6 AIR CHANNEL
[0085] 7 HYDROGEN CHANNEL
* * * * *